The synthesis, properties, and applications of nanotube materials have been extensively studied for more than two decades. Nanotubes are of single-layered or multiple-layered structure wrapped into a cylindrical shape. Nanotubes, especially carbon nanotubes, generally have a diameter of 1 nanometer, but may extend to several micrometers in length. Carbon nanotubes have been found to possess good mechanical, electrical, thermal, and mass transport properties and can be incorporated into other solid phase materials as nanoscale fillers.
The mechanical, electrical or heat/mass transport properties of such nanocomposites exhibit a strong dependence on the filler volume fraction. Previous studies have also suggested that uniformity of the nanotube dispersion in the matrix material plays a critical role in the performance enhancement. For example, carbon nanotube/polymer composite membranes with non-ideal nanotube dispersion (i.e. aggregation of nanotubes in the matrix due to the incompatibility between the outer surface of the carbon nanotubes and the matrix materials) were found to yield low molecular selectivity, where the aggregation may result in nanotube lumps of 100 micrometers in diameter or larger.
Furthermore, the nanotube dispersion and defect areas in carbon nanotube-based composite membranes are not yet fully characterized. As a result, the relative contributions to mass transport from the dispersed nanotubes (pore size <10 nm) and the defect regions (of size ˜1 μm) are unclear.
To address the problem of increasing the nanotube loading in nanocomposites while maintaining good dispersion, a range of techniques for outer surface modification of carbon nanotubes have been developed, such as the use of surfactants or in situ polymerization to enhance the nanotube compatibility with the polymeric matrix. Nevertheless, the highest volume fraction reported to date of carbon nanotubes dispersed in a polymeric material without significant nanotube aggregation is only about 20%. This limitation hinders the performance enhancement that the nanotubes can potentially create in a composite material or membrane.
The limits on carbon nanotube loading in composite materials are likely related to the difficulty of dispersing the nanotubes individually in a liquid prior to preparing the solid-phase composite. However, individual dispersion of nanotubes in polar liquids can be achieved in the case of metal oxide nanotubes that are synthesized hydrothermally or solvothermally and have polar surfaces. Single walled aluminosilicate nanotubes (SWNTs, FIG. 1), which are synthetic analogues of the nanotubular mineral imogolite, can be synthesized hydrothermally with a high degree of dispersion.1-4 These SWNTs are hypothesized to be amenable to the fabrication and application of high-loading nanotube composites with near-ideal dispersion of nanotubes. Furthermore, previous studies have suggested that aluminosilicate SWNTs possess extraordinarily high interior hydrophilicity due to their high inner surface silanol densities, and membranes incorporating them have been predicted to exhibit excellent water transport properties and good water/alcohol selectivity.6-8 The transport properties of these materials can also be controlled by internal surface modification and tuning of the nanotube diameter. Additionally, aluminogermanate nanotubes, a nanomaterial that is structurally analogous to the aluminosilicate nanotube but shorter in length and larger in external diameter, has been surface-modified by octadecylphosphonic acid.12 The surface-modified aluminogermanate nanotube exhibited increase dispersibility in hydrophobic solvents, suggesting potential applications in membranes. Therefore, SWNT/polymer composite membranes are interesting candidates for applications in water/organics separations (e.g., water/ethanol separation as encountered in biofuel production).
Most of current studies focus on the composite materials comprising carbon nanotubes and polymers. For example, US20100160553 discloses a method for dispersing carbon nanotubes in a SWNT/polymer composite, especially by fragmentation of carbon nanotubes by first embedding carbon nanotubes within a catalyst for polymerization, and then performing the polymerization reaction that is known to fracture the catalyst to 0.1 micrometer range. However, such method still has the disadvantage in that only a limited percentage of well-dispersed nanotubes can be included in the composite material (E.g., [0073]: “The SWNT-polymer composite more preferably may comprise up to 0.1%-3% SWNTs”), and therefore limits the physical characteristics of the composite material.
Therefore, there is the need in the art for a method for preparing a well dispersed metal oxide single-wall nanotube SWNT/polymer composite material that has high volume fraction of the SWNT and has enhanced water permeability and/or water/alcohol selectivity.